I have now installed the alarm system. Its a wireless setup which can take up to 16 sensors. Currenlty I have 4 door break sensors and 4 passive PIR and a smoke detector. The company is also devoloping a flood sensor which will be handy for the bilges and can be added to the system.
This is the kit As you can see it does a lot of clever stuff especially the GSM facilities.
Also installed is the 3G wireless router with exteral 9dBi aerial. I hadn't planned on this when I put the lounge wall up. I was rather chuffed with myself that I managed to find and dill into the 2" cavity from outside on the roof on the first attempt. Once this was done it was simple a case of tracing the wire down.
See here for the full spec and Here for the aerial
Both of these devices will run directly from the 12v supply and draw a tiny amount of power. The modem will only be on when we are at the boat, the alarm of course will be on all the time. Standby draw is 0.5a